First things first, delete his number, texts between the two of you, pictures of him, anything you got from him or anything that reminds you of him, and unfriend him from all social media.

Then, write down a list of reasons why you guys didn't work out/his flaws/things he did that annoyed you. Take this list out and read it every time you start missing him and thinking of the good times.

Allow yourself to cry or vent if you need to for the first couple of days. I find that getting all of the tears out of the way early on really helps because eventually you're going to get tired of crying, and even if you want to cry, you're all cried out.

When you're tired of crying and laying in bed all day doing nothing but thinking of him, this is when you can start going out and doing things you enjoy/things to improve yourself. Since it's summer right now, I've been hiking almost every day and that has really helped me with not dwelling on my breakup. You'll still be thinking of him while you're doing these activities but don't let that stop you. Eventually the thoughts of him will lessen.

I know it may be hard to just go do things for yourself when all you're thinking about is him right now. You can think of it this way: you're improving yourself so the next time he sees you, you're going to look a whole lot better. This thought will help motivate you to get out there the first of of days/weeks but eventually when you start to see results, you're going to be doing this more for yourself than him.